Research on the seroepidemiology of the hepatitis B virus (HBV) and hepatitis C virus (HCV) in serially collected serum samples from Miyazaki, Japan, has continued. Two villages are under study, and a third is in the process of being added. One of the villages has a 23% prevalence rate of HCV infection, and studies have shown that the incidence of NEW infections in previously seronegative persons is extremely high, approaching 300/100,000 person-years. Later studies in other villages elsewhere in Japan published by other laboratories have subsequently provided additional support for the concept that pockets of extremely high prevalence of HCV are probably responsible for the high rate of HCV-associated serious liver disease in Japan. This laboratory has been studying the effect of CpG methylation (methylation of cytosine 5'-adjacent to guanosine) of the HBV genes, to determine their role in the virulence of HBV infection and its outcome. CpG methylation is being looked for in the key genes of HBV, including the gene coding for the surface protein (HBsAg) and the core protein (HBcAg), those which form the basis for all of the licensed screening tests applied to whole blood donations at present. To date, serial serum samples from five patients with severe chronic HBV have been studied; CpG methylation was found to be associated with high levels of liver enzymes. In most HBV-infected patients, HBV integration is found at random sites of the host DNA. This has led to the conclusion that insertional mutagenesis does not usually play a role in hepatitis virus associated hepatocarcinogenesis in humans. Reports from several countries have described some HCC patients without detectable HBsAg in whom HBV DNA can be detected in their HCC, liver, or serum. A study was designed to determine whether HBV in such patients is integrated in the host DNA, since a role of HBV in hepatocarcinogenesis in such patients would be more difficult to ascribe if the viral DNA were not integrated. Integration of HBV into the HCC or liver DNA was detected less frequently in HCC patients with HBsAg-negative HBV infections than in HBsAg-positive HCC patients. This could be due to the low titer of HBV DNA that has been reported in HBsAg-negative HBV infections. The Alu-PCR method appeared to be more sensitive in detecting integrated viral DNA than Southern blotting, inverse PCR, and ligation-linker PCR.